1. Place an oven rack in the middle position. Preheat oven to 375 degrees. Spray a 9-by-3-inch round cake pan with vegetable spray. Line a baking sheet with parchment paper or a nonstick silicone baking mat; set aside.

2. Make the sponge cake: Grind the hazelnuts in a food processor with 1/4 cup sugar. In a medium bowl, sift together flour, cornstarch, and hazelnut mixture; set aside.

3. In the bowl of an electric mixer fitted with the whisk attachment, beat the whole eggs, the yolks, remaining 1/2 cup sugar, salt, and almond extract on high speed until thick and pale yellow in color, and mixture holds a thick ribbon when whisk is lifted from bowl, 7 to 8 minutes. Remove bowl from mixer; gently and quickly fold in flour mixture. Gradually drizzle in oil, while folding.

4. Pour batter into prepared cake pan. Set on prepared baking sheet, and bake until a tester comes out clean when inserted into the center of the cake, about 25 to 30 minutes. Remove from oven and turn cake out immediately onto a wire rack. Let cool for at least 1 hour.

5. Make the buttercream: In the heatproof bowl of an electric mixer set over (but not touching) simmering water, whisk egg whites, sugar, and salt until warm to the touch, 1 to 2 minutes. Transfer bowl to mixer fitted with whisk attachment and beat until stiff peaks form. With the mixer running, gradually beat in butter, piece by piece. When all the butter has been added, mixture will break but it will become smooth as you continue to beat. Beat in vanilla. Divide buttercream in half, returning one half to the mixer bowl to make the chocolate flavor and setting aside the remaining half for the praline buttercream.

6. Melt the chocolate in a heatproof bowl set over (but not touching) simmering water. Slowly beat into buttercream in mixer. Add cocoa mixture and beat until buttercream is cooled; set aside.

7. Using a small offset spatula, spread praline paste onto a clean work surface. Smooth paste with a spatula, working out any lumps. Add 1/2 cup reserved buttercream to praline paste and fold it into buttercream. Once the paste and buttercream are fully combined and there are no lumps, fold praline mixture into remaining reserved buttercream.

8. Using a serrated knife, trim top of cake to make level. Cut cake horizontally into even thirds. Place four strips of parchment paper around perimeter of a serving plate or lazy Susan. Place the first layer on the cake plate and top with a 1/3- to 1/2-inch layer of praline buttercream. Place second layer on top; top with remaining praline buttercream. Top with remaining layer.

9. Spread a thin layer of the chocolate buttercream over the tops and sides of the cake to cover, creating a "crumb" layer. Transfer to refrigerator for 15 minutes to set. Spread tops and sides of cake with remaining chocolate buttercream; serve.